A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolus where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, hearing prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system, including an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110.
Typically, the electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104. Depending on context, the electrode contacts 112 are also referred to as electrode channels. In cochlear implants today, a relatively small number of electrode channels are each associated with relatively broad frequency bands, with each electrode contact 112 addressing a group of neurons with an electric stimulation pulse having a charge that is derived from the instantaneous amplitude of the signal envelope within that frequency band.
It is well-known in the field that electric stimulation at different locations within the cochlea produce different frequency percepts. The underlying mechanism in normal acoustic hearing is referred to as the tonotopic principle. In cochlear implant users, the tonotopic organization of the cochlea has been extensively investigated; for example, see Vermeire et al., Neural tonotopy in cochlear implants: An evaluation in unilateral cochlear implant patients with unilateral deafness and tinnitus, Hear Res, 245(1-2), 2008 Sep. 12 p. 98-106; and Schatzer et al., Electric-acoustic pitch comparisons in single-sided-deaf cochlear implant users: Frequency-place functions and rate pitch, Hear Res, 309, 2014 March, p. 26-35 (both of which are incorporated herein by reference in their entireties).
In some stimulation signal coding strategies, stimulation pulses are applied at a constant rate across all electrode channels, whereas in other coding strategies, stimulation pulses are applied at a channel-specific rate. Various specific signal processing schemes can be implemented to produce the electrical stimulation signals. Signal processing approaches that are well-known in the field of cochlear implants include continuous interleaved sampling (CIS), channel specific sampling sequences (CSSS) (as described in U.S. Pat. No. 6,348,070, incorporated herein by reference), spectral peak (SPEAK), and compressed analog (CA) processing.
For an audio prosthesis such as a cochlear implant to work correctly, some patient-specific operating parameters need to be determined in a fit adjustment procedure where the type and number of operating parameters are device dependent and stimulation strategy dependent. Possible patient-specific operating parameters for a cochlear implant include:
THR1 (lower detection threshold of stimulation amplitude) for Electrode 1
MCL1 (maximum comfortable level) for Electrode 1
Phase Duration for Electrode 1
THR2 for Electrode 2
MCL2 for Electrode 2
Phase Duration for Electrode 2
. . .
Pulse Rate (may be also channel dependent)
Number of fine structure channels
Compression
Parameters of frequency->electrode mapping
Parameters describing the electrical field distribution
FIG. 2 shows a block diagram of a cochlear implant fitting system configured to perform such post-implantation fitting. Control Unit 201 for Recording and Stimulation, for example, a Med-El Maestro Cochlear Implant (CI) system, generates stimulation signals and analyzes response measurements. Connected to the Control Unit 201 is an Interface Box 202, for example, a Diagnostic Interface System such as the DIB II conventionally used with the Maestro CI system that formats and distributes the input and output signals between the Control Unit 201 and the system components implanted in the Patient 206. For example, as shown in FIG. 2, there may be an Interface Lead 203 connected at one end to the Interface Box 202 and at the other end having Electrode Plug 207 that then divides into a Cochlear Implant Electrode 204 and an Extra-Cochlear Ground Electrode 205. After delivering a stimulation pulse, a Cochlear Implant Electrode 204 may be used as a sensing element to determine current and voltage characteristics of the adjacent tissue.
One common method for fit adjustment is to behaviorally find the threshold (THR) and maximum comfortable level (MCL) value for each separate electrode contact. See for example, Rätz, Fitting Guide for First Fitting with MAESTRO 2.0, MED-EL, Fürstenweg 77a, 6020 Innsbruck, 1.0 Edition, 2007. AW 5420 Rev. 1.0 (English_EU); incorporated herein by reference. Other alternatives/extensions are sometimes used with a reduced set of operating parameters; e.g. as suggested by Smoorenburg, Cochlear Implant Ear Marks, University Medical Centre Utrecht, 2006; and U.S. Patent Application 20060235332; which are incorporated herein by reference. Typically each stimulation channel is fitted separately without using the information from already fitted channels. The stimulation current on a given electrode typically is increased in steps from zero until the MCL or THR is reached.
FIG. 3 shows a portion of the middle ear anatomy in greater detail, including the incus 301 and the stapes 302. The lenticular process end of the incus 301 vibrates the head 305 of the stapes 302, which in turn vibrates the base 303 of the stapes 302 which couples the vibration into the inner ear (cochlea). Also connected to the head 305 of the stapes 302 is the stapedial tendon 306 of the stapedius muscle situated within the bone of the pyramidal eminence 307. When a loud noise produces an excessively high sound pressure that could damage the inner ear, the stapedius muscle reflexively contracts to decrease the mechanical coupling of the incus 301 to the stapes 302 (and thereby also reduce the force transmission). This protects the inner ear from excessively high sound pressures.
The tensing of the stapedius muscle when triggered by such high sound pressures is also referred to as the stapedius reflex. Medically relevant information about the functional capability of the ear may be obtained by observation of the stapedius reflex. Measurement of the stapedius reflex also is useful for setting and/or calibrating cochlear implants because the threshold of the stapedius reflex is closely correlated to the psychophysical perception of comfortable loudness, the so-called maximum comfortable level (MCL), determined in the fitting process described above.
The stapedius reflex can be determined in an ambulatory clinical setting using an acoustic tympanometer that measures the changes in acoustic impedance of the middle ear caused by stapedial muscle contraction in response to loud sounds. The stiffness of the vibrating elements of a middle ear (also called the compliance) is increased when the stapedius reflex has been elicited. During tympanometry measurements, the outer ear canal is tightly closed by a plug device to define a closed air space between the plug and the tympanic membrane. A tube in the plug provides air from an air pump that is adapted to vary the air pressure within the closed air space relative to the pressure in the middle ear of the patient. The plug also provides a sound source, e.g. a loudspeaker, that is adapted to provide a sound towards the tympanic membrane, and a sensor, e.g. a microphone, that is adapted to sense a reflected portion of the sound provided by the sound source that is reflected from the tympanic membrane. But performing these tests and measurements is rather difficult and requires quite specialized equipment, high skill levels and significant time from the fitting audiologist, as well as full cooperation of the patient.
To measure the stapedius reflex intra-operatively, it also is known to use electrodes that are brought into contact with the stapedius muscle to relay to a measuring device the action current and/or action potentials generated, e.g. a measured EMG signal, upon a contraction of the stapedius muscle. But a reliable minimally-invasive contact of the stapedius muscle is difficult because the stapedius muscle is situated inside the bony pyramidal eminence and only the stapedial tendon is accessible from the interior volume of the middle ear.
Various intraoperative stapedius muscle electrodes are known from U.S. Pat. No. 6,208,882 (incorporated herein by reference in its entirety), however, these only achieve inadequate contact of the stapedius muscle tissue (in particular upon muscle contraction) and are also very traumatizing. This reference describes one embodiment that uses a ball shape monopolar electrode contact with a simple wire attached to it. That would be very difficult to surgically position into a desired position with respect to the stapedius tissue and to fix it there allowing for a long-term atraumatic and stable positioning. Therefore the weakness of this type of electrode is that it does not qualify for chronic implantation. In addition, there is no teaching of how to implement such an arrangement with a bipolar electrode with electrode contacts with sufficient space between each other to enable bipolar registration.
Some intraoperative experiments and studies have been conducted with hook electrodes that have been attached at the stapedius tendon or muscle. These electrode designs were only suitable for acute intra-operative tests. Moreover, some single hook electrodes do not allow a quick and easy placement at the stapedius tendon and muscle—the electrode has to be hand held during intra-operative measurements, while other double hook electrodes do not ensure that both electrodes are inserted into the stapedius muscle due to the small dimensions of the muscle and the flexibility of the electrode tips. One weakness of these intraoperative electrodes is that they do not qualify for chronic implantation.
German patent DE 10 2007 026 645 (incorporated herein by reference in its entirety) discloses a two-part bipolar electrode configuration where a first electrode is pushed onto the stapedius tendon or onto the stapedius muscle itself, and a second electrode is pierced through the first electrode into the stapedius muscle. One disadvantage of the described solution is its rather complicated handling in the very limited space of the surgical operation area, especially manipulation of the fixation electrode. In addition, the piercing depth of the second electrode is not controlled so that trauma can also occur with this approach. Also it is not easy to avoid galvanic contact between both electrodes.
U.S. Patent Publication 20100268054 (incorporated herein by reference in its entirety) describes a different stapedius electrode arrangement having a long support electrode with a base end and a tip for insertion into the target tissue. A fixation electrode also has a base end and a tip at an angle to the electrode body. The tip of the fixation electrode passes perpendicularly through an electrode opening in the support electrode so that the tips of the support and fixation electrodes penetrate into the target tissue so that at least one of the electrodes senses electrical activity in the target stapedius tissue. The disadvantages of this design are analogous to the disadvantages mentioned in the preceding patent.
U.S. Patent Publication 20130281812 (incorporated herein by reference in its entirety) describes a double tile stapedial electrode for bipolar recording. The electrode is configured to be placed over the stapedius tendon and a sharp tip pierces through the bony channel towards the stapedius muscle. The downside of this disclosure is again its rather complicated handling in the very limited space of the surgical operation area,
Various other stapedial electrode designs also are known, all with various associated drawbacks; for example, US 2011/0255731, US 2014/0100471, U.S. Pat. Nos. 8,280,480, and 8,521,250, all incorporated herein by reference in their entireties. A simple wire and ball contact electrode is very difficult to surgically position and to keep it atraumatically stabilized for chronic implantations. The penetrating tip of such a design must be stiff enough to pass through the bone tunnel, but if the tip is too stiff, it is difficult to bend and maneuver the wire into its position. And some stapedius muscle electrode designs are only monopolar electrodes (with a single electrode contact) and are not suitable for a bipolar arrangement with the electrode contacts with sufficient distance between each other to enable bipolar registration. Finally, another design is disclosed in co-pending U.S. Provisional Patent Application 62/105,260 (incorporated herein by reference in its entirety).